Internal combustion engines operating on a six-stroke cycle are generally known in the art. In a six-stroke cycle, a piston reciprocally disposed in a cylinder moves through an intake stroke from a top dead center (TDC) position towards a bottom dead center (BDC) position to admit air or a mixture of air with fuel and/or exhaust gas into the cylinder through one or more intake valves. The intake valve(s) selectively fluidly connect the cylinder with an air source, and are in an open position during the intake stroke to allow the cylinder to fill with air or a mixture thereof.
When the cylinder has sufficiently filled, the intake valve(s) close(s) to fluidly trap the air or air mixture within the cylinder. During a compression stroke, the piston moves back towards the TDC position to compress the air or the air mixture trapped in the cylinder. During this process, an initial or additional fuel charge may be introduced to the cylinder by an injector. The compressed air/fuel mixture in the cylinder then ignites, thus increasing fluid pressure within the cylinder. The increased pressure pushes the piston towards the BDC position in what is commonly referred to as a combustion or power stroke.
In accordance with the six-stroke cycle, the piston performs a second compression stroke in which it recompresses the combustion products remaining in the cylinder after the first combustion or power stroke. During this recompression, any exhaust valves associated with the cylinder remain generally closed to assist cylinder recompression. Optionally, a second fuel charge and/or additional air may be introduced into the cylinder during recompression to assist igniting the residual combustion products and produce a second power stroke. Following the second power stroke, the cylinder undergoes an exhaust stroke during which the piston moves towards the TDC position and one or more exhaust valves are opened to help evacuate combustion by-products from the cylinder.
One example of an internal combustion engine configured to operate on a six-stroke engine can be found in U.S. Pat. No. 7,418,928. This disclosure relates to a method of operating an engine that includes compressing part of the combustion gas after a first combustion stroke of the piston as well as an additional combustion stroke during a six-stroke cycle of the engine.
The re-compression and re-combustion of combustion products from the first power stroke of a cylinder in six-stroke engines, however, often results in increased emissions, and especially emissions that result when the fluids within the cylinder are at a high temperature. For example, the production of nitrous oxides (NOx) increases with increasing cylinder temperatures. The production of such and other emissions is disfavored, especially since NOx emissions are regulated for diesel engines.